Efficient cache management in a tiled architecture

ABSTRACT

A surface cache stores pixel data on behalf of a pixel processing pipeline that is configured to generate screen tiles. The surface cache assigns hint levels to cache lines storing pixel data according to whether that pixel data is likely to be needed again. When the pixel data is needed to process a subsequent tile, the corresponding cache line is assigned a higher hint value. When the pixel data is not needed again, the corresponding cache line is assigned a lower hint value. The surface cache is configured to preferentially evict cache lines having a lower hint value, thereby preserving cache lines that store pixel data needed for future processing. In addition, a fetch controller is configured to throttle the rate at which fetch requests are issued to the surface cache to prevent situations where pixel data needed for future operations becomes prematurely evicted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to caching and,more specifically, to efficient cache management in a tiledarchitecture.

2. Description of the Related Art

A conventional cache unit typically stores data that is to be processedby a hardware element or collection of hardware elements. Since thecache unit resides physically close to the hardware element(s), memorybandwidth may be improved since memory requests for data already storedin the cache need not be transmitted to global memory. This basiccaching technique is ubiquitous throughout most modern computer systems.

In a computer system configured for graphics processing, cache unitsoftentimes store graphics data to be processed by different types ofgraphics processing pipelines. For example, a pixel cache unit that iscoupled to a pixel processing pipeline could store pixel data to beprocessed for display on a display screen. In such a case, the pixelcache may improve memory bandwidth because the pixel processing pipelinemay need to access the same portion of pixel data multiple differenttimes when rendering an image for display.

In a tiled architecture, the pixel processing pipeline may process pixeldata associated with neighboring screen tiles that could potentiallyoverlap with one another, and the pixel cache could store pixel dataassociated with the overlapping region. With this approach, pixel dataassociated with an overlapping region shared between first and secondscreen tiles would only need to be accessed from global memory once(e.g., when the first screen tile is processed), and would be cached forlater use when needed again (e.g., when the second screen tile isprocessed). This approach provides reasonable benefits in simplesituations such as that described herein.

However, other situations may arise where the above approach causessignificant thrashing of the pixel cache. Returning to the previousexample, the pixel data associated with the overlapping region betweenthe first and second screen tiles may also be needed by a tenth screentile that overlaps the first screen tile. By the time the pixelprocessing pipeline has processed the third through ninth screen tiles,the pixel data shared between the first and tenth screen tiles may havealready been evicted from the pixel cache to create room for pixel dataassociated with the intervening screen tiles. The pixel processingpipeline would then need to re-request the evicted pixel data fromglobal memory, thereby thrashing the pixel cache to a certain degree.Depending on the size of the screen tiles and the order in which thosetiles are processed, such thrashing may have a significant impact on theefficiency of the pixel cache.

Accordingly, what is needed in the art is a more efficient technique forcaching pixel data in a tiled architecture.

SUMMARY OF THE INVENTION

One embodiment of the present invention sets forth a subsystemconfigured to cache data in a tile-based architecture, including a cacheunit configured to receive a request for a first portion of datacorresponding to a first subregion associated with a screen tile,determine that the first portion of data is not resident in the cacheunit, acquire the first portion of data from a memory unit, identify,based on a cache policy, a first cache line in the cache unit thatstores a second portion of data corresponding to a second subregionassociated with the screen tile, where the cache policy indicates thatone or more portions of data corresponding to the second subregionshould be evicted before one or more portions of data corresponding toother subregions associated with the screen tile are evicted, evict thesecond portion of data from the first cache line, and, after evicting,store the first portion of data in the first cache line.

One advantage of the disclosed technique is that cache efficiency isimproved because data that is needed again is retained within the cache.Another advantage is that additional data is not fetched until cachelines that are no longer necessary are unlocked, thereby avoidingsituations where data needed for future operations is prematurelyevicted.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram illustrating a computer system configured toimplement one or more aspects of the present invention;

FIG. 2 is a block diagram of a parallel processing unit included in theparallel processing subsystem of FIG. 1, according to one embodiment ofthe present invention;

FIG. 3A is a block diagram of a general processing cluster included inthe parallel processing unit of FIG. 2, according to one embodiment ofthe present invention;

FIG. 3B is a conceptual diagram of a graphics processing pipeline thatmay be implemented within the parallel processing unit of FIG. 2,according to one embodiment of the present invention;

FIG. 4 is a conceptual diagram of a cache tile that the graphicsprocessing pipeline of FIG. 3B may be configured to generate andprocess, according to one embodiment of the present invention;

FIG. 5 is a block diagram of a subsystem configured to process pixeldata to generate pixels for display, according to one embodiment of thepresent invention;

FIG. 6A is a conceptual diagram illustrating a pattern for accessingpixel data associated with a cache tile, according to one embodiment ofthe present invention;

FIG. 6B is a conceptual diagram illustrating a technique for cachingpixel data associated with a screen tile, according to one embodiment ofthe present invention;

FIG. 7 is a flow diagram of method steps for throttling fetch requestsissued to a surface cache, according to one embodiment of the presentinvention; and

FIG. 8 is a flow diagram is method steps for managing pixel data in asurface cache during processing of a screen tile, according to oneembodiment of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the present invention. However,it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails.

System Overview

FIG. 1 is a block diagram illustrating a computer system 100 configuredto implement one or more aspects of the present invention. As shown,computer system 100 includes, without limitation, a central processingunit (CPU) 102 and a system memory 104 coupled to a parallel processingsubsystem 112 via a memory bridge 105 and a communication path 113.Memory bridge 105 is further coupled to an I/O (input/output) bridge 107via a communication path 106, and I/O bridge 107 is, in turn, coupled toa switch 116.

In operation, I/O bridge 107 is configured to receive user inputinformation from input devices 108, such as a keyboard or a mouse, andforward the input information to CPU 102 for processing viacommunication path 106 and memory bridge 105. Switch 116 is configuredto provide connections between I/O bridge 107 and other components ofthe computer system 100, such as a network adapter 118 and variousadd-in cards 120 and 121.

As also shown, I/O bridge 107 is coupled to a system disk 114 that maybe configured to store content and applications and data for use by CPU102 and parallel processing subsystem 112. As a general matter, systemdisk 114 provides non-volatile storage for applications and data and mayinclude fixed or removable hard disk drives, flash memory devices, andCD-ROM (compact disc read-only-memory), DVD-ROM (digital versatiledisc-ROM), Blu-ray, HD-DVD (high definition DVD), or other magnetic,optical, or solid state storage devices. Finally, although notexplicitly shown, other components, such as universal serial bus orother port connections, compact disc drives, digital versatile discdrives, film recording devices, and the like, may be connected to I/Obridge 107 as well.

In various embodiments, memory bridge 105 may be a Northbridge chip, andI/O bridge 107 may be a Southbridge chip. In addition, communicationpaths 106 and 113, as well as other communication paths within computersystem 100, may be implemented using any technically suitable protocols,including, without limitation, AGP (Accelerated Graphics Port),HyperTransport, or any other bus or point-to-point communicationprotocol known in the art.

In some embodiments, parallel processing subsystem 112 comprises agraphics subsystem that delivers pixels to a display device 110 that maybe any conventional cathode ray tube, liquid crystal display,light-emitting diode display, or the like. In such embodiments, theparallel processing subsystem 112 incorporates circuitry optimized forgraphics and video processing, including, for example, video outputcircuitry. As described in greater detail below in FIG. 2, suchcircuitry may be incorporated across one or more parallel processingunits (PPUs) included within parallel processing subsystem 112. In otherembodiments, the parallel processing subsystem 112 incorporatescircuitry optimized for general purpose and/or compute processing.Again, such circuitry may be incorporated across one or more PPUsincluded within parallel processing subsystem 112 that are configured toperform such general purpose and/or compute operations. In yet otherembodiments, the one or more PPUs included within parallel processingsubsystem 112 may be configured to perform graphics processing, generalpurpose processing, and compute processing operations. System memory 104includes at least one device driver 103 configured to manage theprocessing operations of the one or more PPUs within parallel processingsubsystem 112.

In various embodiments, parallel processing subsystem 112 may beintegrated with one or more other the other elements of FIG. 1 to form asingle system. For example, parallel processing subsystem 112 may beintegrated with CPU 102 and other connection circuitry on a single chipto form a system on chip (SoC).

It will be appreciated that the system shown herein is illustrative andthat variations and modifications are possible. The connection topology,including the number and arrangement of bridges, the number of CPUs 102,and the number of parallel processing subsystems 112, may be modified asdesired. For example, in some embodiments, system memory 104 could beconnected to CPU 102 directly rather than through memory bridge 105, andother devices would communicate with system memory 104 via memory bridge105 and CPU 102. In other alternative topologies, parallel processingsubsystem 112 may be connected to I/O bridge 107 or directly to CPU 102,rather than to memory bridge 105. In still other embodiments, I/O bridge107 and memory bridge 105 may be integrated into a single chip insteadof existing as one or more discrete devices. Lastly, in certainembodiments, one or more components shown in FIG. 1 may not be present.For example, switch 116 could be eliminated, and network adapter 118 andadd-in cards 120, 121 would connect directly to I/O bridge 107.

FIG. 2 is a block diagram of a parallel processing unit (PPU) 202included in the parallel processing subsystem 112 of FIG. 1, accordingto one embodiment of the present invention. Although FIG. 2 depicts onePPU 202, as indicated above, parallel processing subsystem 112 mayinclude any number of PPUs 202. As shown, PPU 202 is coupled to a localparallel processing (PP) memory 204. PPU 202 and PP memory 204 may beimplemented using one or more integrated circuit devices, such asprogrammable processors, application specific integrated circuits(ASICs), or memory devices, or in any other technically feasiblefashion.

In some embodiments, PPU 202 comprises a graphics processing unit (GPU)that may be configured to implement a graphics rendering pipeline toperform various operations related to generating pixel data based ongraphics data supplied by CPU 102 and/or system memory 104. Whenprocessing graphics data, PP memory 204 can be used as graphics memorythat stores one or more conventional frame buffers and, if needed, oneor more other render targets as well. Among other things, PP memory 204may be used to store and update pixel data and deliver final pixel dataor display frames to display device 110 for display. In someembodiments, PPU 202 also may be configured for general-purposeprocessing and compute operations.

In operation, CPU 102 is the master processor of computer system 100,controlling and coordinating operations of other system components. Inparticular, CPU 102 issues commands that control the operation of PPU202. In some embodiments, CPU 102 writes a stream of commands for PPU202 to a data structure (not explicitly shown in either FIG. 1 or FIG.2) that may be located in system memory 104, PP memory 204, or anotherstorage location accessible to both CPU 102 and PPU 202. A pointer tothe data structure is written to a pushbuffer to initiate processing ofthe stream of commands in the data structure. The PPU 202 reads commandstreams from the pushbuffer and then executes commands asynchronouslyrelative to the operation of CPU 102. In embodiments where multiplepushbuffers are generated, execution priorities may be specified foreach pushbuffer by an application program via device driver 103 tocontrol scheduling of the different pushbuffers.

As also shown, PPU 202 includes an I/O (input/output) unit 205 thatcommunicates with the rest of computer system 100 via the communicationpath 113 and memory bridge 105. I/O unit 205 generates packets (or othersignals) for transmission on communication path 113 and also receivesall incoming packets (or other signals) from communication path 113,directing the incoming packets to appropriate components of PPU 202. Forexample, commands related to processing tasks may be directed to a hostinterface 206, while commands related to memory operations (e.g.,reading from or writing to PP memory 204) may be directed to a crossbarunit 210. Host interface 206 reads each pushbuffer and transmits thecommand stream stored in the pushbuffer to a front end 212.

As mentioned above in conjunction with FIG. 1, the connection of PPU 202to the rest of computer system 100 may be varied. In some embodiments,parallel processing subsystem 112, which includes at least one PPU 202,is implemented as an add-in card that can be inserted into an expansionslot of computer system 100. In other embodiments, PPU 202 can beintegrated on a single chip with a bus bridge, such as memory bridge 105or I/O bridge 107. Again, in still other embodiments, some or all of theelements of PPU 202 may be included along with CPU 102 in a singleintegrated circuit or system of chip (SoC).

In operation, front end 212 transmits processing tasks received fromhost interface 206 to a work distribution unit (not shown) withintask/work unit 207. The work distribution unit receives pointers toprocessing tasks that are encoded as task metadata (TMD) and stored inmemory. The pointers to TMDs are included in a command stream that isstored as a pushbuffer and received by the front end unit 212 from thehost interface 206. Processing tasks that may be encoded as TMDs includeindices associated with the data to be processed as well as stateparameters and commands that define how the data is to be processed. Forexample, the state parameters and commands could define the program tobe executed on the data. The task/work unit 207 receives tasks from thefront end 212 and ensures that GPCs 208 are configured to a valid statebefore the processing task specified by each one of the TMDs isinitiated. A priority may be specified for each TMD that is used toschedule the execution of the processing task. Processing tasks also maybe received from the processing cluster array 230. Optionally, the TMDmay include a parameter that controls whether the TMD is added to thehead or the tail of a list of processing tasks (or to a list of pointersto the processing tasks), thereby providing another level of controlover execution priority.

PPU 202 advantageously implements a highly parallel processingarchitecture based on a processing cluster array 230 that includes a setof C general processing clusters (GPCs) 208, where C≧1. Each GPC 208 iscapable of executing a large number (e.g., hundreds or thousands) ofthreads concurrently, where each thread is an instance of a program. Invarious applications, different GPCs 208 may be allocated for processingdifferent types of programs or for performing different types ofcomputations. The allocation of GPCs 208 may vary depending on theworkload arising for each type of program or computation.

Memory interface 214 includes a set of D of partition units 215, whereD≧1. Each partition unit 215 is coupled to one or more dynamic randomaccess memories (DRAMs) 220 residing within PPM memory 204. In oneembodiment, the number of partition units 215 equals the number of DRAMs220, and each partition unit 215 is coupled to a different DRAM 220. Inother embodiments, the number of partition units 215 may be differentthan the number of DRAMs 220. Persons of ordinary skill in the art willappreciate that a DRAM 220 may be replaced with any other technicallysuitable storage device. In operation, various render targets, such astexture maps and frame buffers, may be stored across DRAMs 220, allowingpartition units 215 to write portions of each render target in parallelto efficiently use the available bandwidth of PP memory 204.

A given GPCs 208 may process data to be written to any of the DRAMs 220within PP memory 204. Crossbar unit 210 is configured to route theoutput of each GPC 208 to the input of any partition unit 215 or to anyother GPC 208 for further processing. GPCs 208 communicate with memoryinterface 214 via crossbar unit 210 to read from or write to variousDRAMs 220. In one embodiment, crossbar unit 210 has a connection to I/Ounit 205, in addition to a connection to PP memory 204 via memoryinterface 214, thereby enabling the processing cores within thedifferent GPCs 208 to communicate with system memory 104 or other memorynot local to PPU 202. In the embodiment of FIG. 2, crossbar unit 210 isdirectly connected with I/O unit 205. In various embodiments, crossbarunit 210 may use virtual channels to separate traffic streams betweenthe GPCs 208 and partition units 215.

Again, GPCs 208 can be programmed to execute processing tasks relatingto a wide variety of applications, including, without limitation, linearand nonlinear data transforms, filtering of video and/or audio data,modeling operations (e.g., applying laws of physics to determineposition, velocity and other attributes of objects), image renderingoperations (e.g., tessellation shader, vertex shader, geometry shader,and/or pixel/fragment shader programs), general compute operations, etc.In operation, PPU 202 is configured to transfer data from system memory104 and/or PP memory 204 to one or more on-chip memory units, processthe data, and write result data back to system memory 104 and/or PPmemory 204. The result data may then be accessed by other systemcomponents, including CPU 102, another PPU 202 within parallelprocessing subsystem 112, or another parallel processing subsystem 112within computer system 100.

As noted above, any number of PPUs 202 may be included in a parallelprocessing subsystem 112. For example, multiple PPUs 202 may be providedon a single add-in card, or multiple add-in cards may be connected tocommunication path 113, or one or more of PPUs 202 may be integratedinto a bridge chip. PPUs 202 in a multi-PPU system may be identical toor different from one another. For example, different PPUs 202 mighthave different numbers of processing cores and/or different amounts ofPP memory 204. In implementations where multiple PPUs 202 are present,those PPUs may be operated in parallel to process data at a higherthroughput than is possible with a single PPU 202. Systems incorporatingone or more PPUs 202 may be implemented in a variety of configurationsand form factors, including, without limitation, desktops, laptops,handheld personal computers or other handheld devices, servers,workstations, game consoles, embedded systems, and the like.

FIG. 3A is a block diagram of a GPC 208 included in PPU 202 of FIG. 2,according to one embodiment of the present invention. In operation, GPC208 may be configured to execute a large number of threads in parallelto perform graphics, general processing and/or compute operations. Asused herein, a “thread” refers to an instance of a particular programexecuting on a particular set of input data. In some embodiments,single-instruction, multiple-data (SIMD) instruction issue techniquesare used to support parallel execution of a large number of threadswithout providing multiple independent instruction units. In otherembodiments, single-instruction, multiple-thread (SIMT) techniques areused to support parallel execution of a large number of generallysynchronized threads, using a common instruction unit configured toissue instructions to a set of processing engines within GPC 208. Unlikea SIMD execution regime, where all processing engines typically executeidentical instructions, SIMT execution allows different threads to morereadily follow divergent execution paths through a given program.Persons of ordinary skill in the art will understand that a SIMDprocessing regime represents a functional subset of a SIMT processingregime.

Operation of GPC 208 is controlled via a pipeline manager 305 thatdistributes processing tasks received from a work distribution unit (notshown) within task/work unit 207 to one or more streamingmultiprocessors (SMs) 310. Pipeline manager 305 may also be configuredto control a work distribution crossbar 330 by specifying destinationsfor processed data output by SMs 310.

In one embodiment, GPC 208 includes a set of M of SMs 310, where M≧1.Also, each SM 310 includes a set of functional execution units (notshown), such as execution units and load-store units. Processingoperations specific to any of the functional execution units may bepipelined, which enables a new instruction to be issued for executionbefore a previous instruction has completed execution. Any combinationof functional execution units within a given SM 310 may be provided. Invarious embodiments, the functional execution units may be configured tosupport a variety of different operations including integer and floatingpoint arithmetic (e.g., addition and multiplication), comparisonoperations, Boolean operations (AND, OR, XOR), bit-shifting, andcomputation of various algebraic functions (e.g., planar interpolationand trigonometric, exponential, and logarithmic functions, etc.).Advantageously, the same functional execution unit can be configured toperform different operations.

In operation, each SM 310 is configured to process one or more threadgroups. As used herein, a “thread group” or “warp” refers to a group ofthreads concurrently executing the same program on different input data,with one thread of the group being assigned to a different executionunit within an SM 310. A thread group may include fewer threads than thenumber of execution units within the SM 310, in which case some of theexecution may be idle during cycles when that thread group is beingprocessed. A thread group may also include more threads than the numberof execution units within the SM 310, in which case processing may occurover consecutive clock cycles. Since each SM 310 can support up to Gthread groups concurrently, it follows that up to G*M thread groups canbe executing in GPC 208 at any given time.

Additionally, a plurality of related thread groups may be active (indifferent phases of execution) at the same time within an SM 310. Thiscollection of thread groups is referred to herein as a “cooperativethread array” (“CTA”) or “thread array.” The size of a particular CTA isequal to m*k, where k is the number of concurrently executing threads ina thread group, which is typically an integer multiple of the number ofexecution units within the SM 310, and m is the number of thread groupssimultaneously active within the SM 310.

Although not shown in FIG. 3A, each SM 310 contains a level one (L1)cache or uses space in a corresponding L1 cache outside of the SM 310 tosupport, among other things, load and store operations performed by theexecution units. Each SM 310 also has access to level two (L2) caches(not shown) that are shared among all GPCs 208 in PPU 202. The L2 cachesmay be used to transfer data between threads. Finally, SMs 310 also haveaccess to off-chip “global” memory, which may include PP memory 204and/or system memory 104. It is to be understood that any memoryexternal to PPU 202 may be used as global memory. Additionally, as shownin FIG. 3A, a level one-point-five (L1.5) cache 335 may be includedwithin GPC 208 and configured to receive and hold data requested frommemory via memory interface 214 by SM 310. Such data may include,without limitation, instructions, uniform data, and constant data. Inembodiments having multiple SMs 310 within GPC 208, the SMs 310 maybeneficially share common instructions and data cached in L1.5 cache335.

Each GPC 208 may have an associated memory management unit (MMU) 320that is configured to map virtual addresses into physical addresses. Invarious embodiments, MMU 320 may reside either within GPC 208 or withinthe memory interface 214. The MMU 320 includes a set of page tableentries (PTEs) used to map a virtual address to a physical address of atile or memory page and optionally a cache line index. The MMU 320 mayinclude address translation lookaside buffers (TLB) or caches that mayreside within SMs 310, within one or more L1 caches, or within GPC 208.

In graphics and compute applications, GPC 208 may be configured suchthat each SM 310 is coupled to a texture unit 315 for performing texturemapping operations, such as determining texture sample positions,reading texture data, and filtering texture data.

In operation, each SM 310 transmits a processed task to workdistribution crossbar 330 in order to provide the processed task toanother GPC 208 for further processing or to store the processed task inan L2 cache (not shown), parallel processing memory 204, or systemmemory 104 via crossbar unit 210. In addition, a pre-raster operations(preROP) unit 325 is configured to receive data from SM 310, direct datato one or more raster operations (ROP) units within partition units 215,perform optimizations for color blending, organize pixel color data, andperform address translations.

It will be appreciated that the core architecture described herein isillustrative and that variations and modifications are possible. Amongother things, any number of processing units, such as SMs 310, textureunits 315, or preROP units 325, may be included within GPC 208. Further,as described above in conjunction with FIG. 2, PPU 202 may include anynumber of GPCs 208 that are configured to be functionally similar to oneanother so that execution behavior does not depend on which GPC 208receives a particular processing task. Further, each GPC 208 operatesindependently of the other GPCs 208 in PPU 202 to execute tasks for oneor more application programs. In view of the foregoing, persons ofordinary skill in the art will appreciate that the architecturedescribed in FIGS. 1-3A in no way limits the scope of the presentinvention.

Graphics Pipeline Architecture

FIG. 3B is a conceptual diagram of a graphics processing pipeline 350that may be implemented within PPU 202 of FIG. 2, according to oneembodiment of the present invention. As shown, the graphics processingpipeline 350 includes, without limitation, a primitive distributor (PD)355; a vertex attribute fetch unit (VAF) 360; a vertex, tessellation,geometry processing unit (VTG) 365; a viewport scale, cull, and clipunit (VPC) 370; a tiling unit 375, a setup unit (setup) 380, arasterizer (raster) 385; a fragment processing unit, also identified asa pixel shading unit (PS) 390, and a raster operations unit (ROP) 395.

The PD 355 collects vertex data associated with high-order surfaces,graphics primitives, and the like, from the front end 212 and transmitsthe vertex data to the VAF 360.

The VAF 360 retrieves vertex attributes associated with each of theincoming vertices from shared memory and stores the vertex data, alongwith the associated vertex attributes, into shared memory.

The VTG 365 is a programmable execution unit that is configured toexecute vertex shader programs, tessellation programs, and geometryprograms. These programs process the vertex data and vertex attributesreceived from the VAF 360 and produce graphics primitives for furtherprocessing within the graphics processing pipeline 350. Although notexplicitly shown, the VTG 365 may include, in some embodiments, one ormore of a vertex processing unit, a tessellation initializationprocessing unit, a task generation unit, a task distributor, a topologygeneration unit, a tessellation processing unit, and a geometryprocessing unit.

The vertex processing unit is a programmable execution unit that isconfigured to execute vertex shader programs, lighting and transformingvertex data as specified by the vertex shader programs. For example, thevertex processing unit may be programmed to transform the vertex datafrom an object-based coordinate representation (object space) to analternatively based coordinate system such as world space or normalizeddevice coordinates (NDC) space. The vertex processing unit may readvertex data and vertex attributes that is stored in shared memory by theVAF and may process the vertex data and vertex attributes. The vertexprocessing unit 415 stores processed vertices in shared memory.

The tessellation initialization processing unit is a programmableexecution unit that is configured to execute tessellation initializationshader programs. The tessellation initialization processing unitprocesses vertices produced by the vertex processing unit and generatesgraphics primitives known as patches. The tessellation initializationprocessing unit also generates various patch attributes. Thetessellation initialization processing unit then stores the patch dataand patch attributes in shared memory. In some embodiments, thetessellation initialization shader program may be called a hull shaderor a tessellation control shader.

The task generation unit retrieves data and attributes for vertices andpatches from shared memory. The task generation unit generates tasks forprocessing the vertices and patches for processing by later stages inthe graphics processing pipeline 350.

The task distributor redistributes the tasks produced by the taskgeneration unit. The tasks produced by the various instances of thevertex shader program and the tessellation initialization program mayvary significantly between one graphics processing pipeline 350 andanother. The task distributor redistributes these tasks such that eachgraphics processing pipeline 350 has approximately the same workloadduring later pipeline stages.

The topology generation unit retrieves tasks distributed by the taskdistributor. The topology generation unit indexes the vertices,including vertices associated with patches, and computes (U,V)coordinates for tessellation vertices and the indices that connect thetessellated vertices to form graphics primitives. The topologygeneration unit then stores the indexed vertices in shared memory.

The tessellation processing unit is a programmable execution unit thatis configured to execute tessellation shader programs. The tessellationprocessing unit reads input data from and writes output data to sharedmemory. This output data in shared memory is passed to the next shaderstage, the geometry processing unit 445 as input data. In someembodiments, the tessellation shader program may be called a domainshader or a tessellation evaluation shader.

The geometry processing unit is a programmable execution unit that isconfigured to execute geometry shader programs, thereby transforminggraphics primitives. Vertices are grouped to construct graphicsprimitives for processing, where graphics primitives include triangles,line segments, points, and the like. For example, the geometryprocessing unit may be programmed to subdivide the graphics primitivesinto one or more new graphics primitives and calculate parameters, suchas plane equation coefficients, that are used to rasterize the newgraphics primitives.

The geometry processing unit transmits the parameters and verticesspecifying new graphics primitives to the VPC 370. The geometryprocessing unit may read data that is stored in shared memory for use inprocessing the geometry data. The VPC 370 performs clipping, culling,and viewport transform to determine which graphics primitives arepotentially viewable in the final rendered image and which graphicsprimitives are not potentially viewable. The VPC 370 then transmitsprocessed graphics primitives to the tiling unit 375.

The tiling unit 375 is a graphics primitive sorting engine that residesbetween a world space pipeline 352 and a screen space pipeline 354, asfurther described herein. Graphics primitives are processed in the worldspace pipeline 352 and then transmitted to the tiling unit 375. Thescreen space is divided into cache tiles, where each cache tile isassociated with a portion of the screen space. For each graphicsprimitive, the tiling unit 375 identifies the set of cache tiles thatintersect with the graphics primitive, a process referred to herein as“tiling.” After tiling a certain number of graphics primitives, thetiling unit 375 processes the graphics primitives on a cache tile basis,where graphics primitives associated with a particular cache tile aretransmitted to the setup unit 380. The tiling unit 375 transmitsgraphics primitives to the setup unit 380 one cache tile at a time.Graphics primitives that intersect with multiple cache tiles aretypically processed once in the world space pipeline 352, but are thentransmitted multiple times to the screen space pipeline 354.

Such a technique improves cache memory locality during processing in thescreen space pipeline 354, where multiple memory operations associatedwith a first cache tile access a region of the L2 caches, or any othertechnically feasible cache memory, that may stay resident during screenspace processing of the first cache tile. Once the graphics primitivesassociated with the first cache tile are processed by the screen spacepipeline 354, the portion of the L2 caches associated with the firstcache tile may be flushed and the tiling unit may transmit graphicsprimitives associated with a second cache tile. Multiple memoryoperations associated with a second cache tile may then access theregion of the L2 caches that may stay resident during screen spaceprocessing of the second cache tile. Accordingly, the overall memorytraffic to the L2 caches and to the render targets may be reduced. Insome embodiments, the world space computation is performed once for agiven graphics primitive irrespective of the number of cache tiles inscreen space that intersects with the graphics primitive.

The setup unit 380 receives vertex data from the VPC 370 via the tilingunit 375 and calculates parameters associated with the graphicsprimitives, including, without limitation, the color values, surfacenormal vectors, and transparency values at each vertex of the graphicsprimitive. The setup unit 380 then transmits processed graphicsprimitives to rasterizer 385.

The rasterizer 385 scan converts the new graphics primitives andtransmits fragments and coverage data to the pixel shading unit 390.Additionally, the rasterizer 385 may be configured to perform z cullingand other z-based optimizations.

The pixel shading unit 390 is a programmable execution unit that isconfigured to execute fragment shader programs, transforming fragmentsreceived from the rasterizer 385, as specified by the fragment shaderprograms. Fragment shader programs may shade fragments at pixel-levelgranularity, where such shader programs may be called pixel shaderprograms. Alternatively, fragment shader programs may shade fragments atsample-level granularity, where each pixel includes multiple samples,and each sample represents a portion of a pixel. Alternatively, fragmentshader programs may shade fragments at any other technically feasiblegranularity, depending on the programmed sampling rate.

In various embodiments, the fragment processing unit 460 may beprogrammed to perform operations such as perspective correction, texturemapping, shading, blending, and the like, to produce shaded fragmentsthat are transmitted to the ROP 395. The pixel shading unit 390 may readdata that is stored in shared memory.

The ROP 395 is a processing unit that performs raster operations, suchas stencil, z test, blending, and the like, and transmits pixel data asprocessed graphics data for storage in graphics memory via the memoryinterface 214, where graphics memory is typically structured as one ormore render targets. The processed graphics data may be stored ingraphics memory, parallel processing memory 204, or system memory 104for display on display device 110 or for further processing by CPU 102or parallel processing subsystem 112. In some embodiments, the ROP 395is configured to compress z or color data that is written to memory anddecompress z or color data that is read from memory. In variousembodiments, the ROP 395 may be located in the memory interface 214, inthe GPCs 208, in the processing cluster array 230 outside of the GPCs,or in a separate unit (not shown) within the PPUs 202.

The graphics processing pipeline may be implemented by any one or moreprocessing elements within PPU 202. For example, one of the SMs 310 ofFIG. 3A could be configured to perform the functions of one or more ofthe VTG 365 and the pixel shading unit 390. The functions of the PD 355,the VAF 360, the VPC 450, the tiling unit 375, the setup unit 380, therasterizer 385, and the ROP 395 may also be performed by processingelements within a particular GPC 208 in conjunction with a correspondingpartition unit 215. Alternatively, graphics processing pipeline 350 maybe implemented using dedicated fixed-function processing elements forone or more of the functions listed above. In various embodiments, PPU202 may be configured to implement one or more graphics processingpipelines 350.

In some embodiments, the graphics processing pipeline 350 may be dividedinto a world space pipeline 352 and a screen space pipeline 354. Theworld space pipeline 352 processes graphics objects in 3D space, wherethe position of each graphics object is known relative to other graphicsobjects and relative to a 3D coordinate system. The screen spacepipeline 354 processes graphics objects that have been projected fromthe 3D coordinate system onto a 2D planar surface representing thesurface of the display device 110. For example, the world space pipeline352 could include pipeline stages in the graphics processing pipeline350 from the PD 355 through the VPC 370. The screen space pipeline 354could include pipeline stages in the graphics processing pipeline 350from the setup unit 380 through the ROP 395. The tiling unit 375 wouldfollow the last stage of the world space pipeline 352, namely, the VPC370. The tiling unit 375 would precede the first stage of the screenspace pipeline 354, namely, the setup unit 380.

In some embodiments, the world space pipeline 352 may be further dividedinto an alpha phase pipeline and a beta phase pipeline. For example, thealpha phase pipeline could include pipeline stages in the graphicsprocessing pipeline 350 from the PD 355 through the task generationunit. The beta phase pipeline could include pipeline stages in thegraphics processing pipeline 350 from the topology generation unitthrough the VPC 370. The graphics processing pipeline 350 performs afirst set of operations during processing in the alpha phase pipelineand a second set of operations during processing in the beta phasepipeline. As used herein, a set of operations is defined as one or moreinstructions executed by a single thread, by a thread group, or bymultiple thread groups acting in unison.

In a system with multiple graphics processing pipeline 350, the vertexdata and vertex attributes associated with a set of graphics objects maybe divided so that each graphics processing pipeline 350 hasapproximately the same amount of workload through the alpha phase. Alphaphase processing may significantly expand the amount of vertex data andvertex attributes, such that the amount of vertex data and vertexattributes produced by the task generation unit is significantly largerthan the amount of vertex data and vertex attributes processed by the PD355 and VAF 360. Further, the task generation unit associated with onegraphics processing pipeline 350 may produce a significantly greaterquantity of vertex data and vertex attributes than the task generationunit associated with another graphics processing pipeline 350, even incases where the two graphics processing pipelines 350 process the samequantity of attributes at the beginning of the alpha phase pipeline. Insuch cases, the task distributor redistributes the attributes producedby the alpha phase pipeline such that each graphics processing pipeline350 has approximately the same workload at the beginning of the betaphase pipeline.

Please note, as used herein, references to shared memory may include anyone or more technically feasible memories, including, withoutlimitation, a local memory shared by one or more SMs 310, or a memoryaccessible via the memory interface 214, such as a cache memory,parallel processing memory 204, or system memory 104. Please also note,as used herein, references to cache memory may include any one or moretechnically feasible memories, including, without limitation, an L1cache, an L1.5 cache, and the L2 caches.

Tiled Caching

FIG. 4 is a conceptual diagram of a cache tile 410(0) that the graphicsprocessing pipeline 350 of FIG. 3B may be configured to generate andprocess, according to one embodiment of the present invention. As shown,the cache tile 410(0) represents a portion of a screen space 400 and isdivided into multiple raster tiles 420.

The screen space 400 represents one or more memory buffers configured tostore rendered image data and other data transmitted by functional unitswithin the graphics processing pipeline 350. In some embodiments, theone or more memory buffers may be configured as one or more rendertargets. The screen space represents a memory buffer configured to storethe image rendered by the graphics processing pipeline. The screen space400 may be associated with any number of render targets, where eachrender target may be configured independently of other render targets toinclude any number of fields. Each field within a render target may beconfigured independently of other fields to include any number of bits.Each render target may include multiple picture elements (pixels), andeach pixel may, in turn, include multiple samples. In some embodiments,the size of each cache tile may be based on the size and configurationof the render targets associated with the screen space. In operation,once rendering completes, the pixels in the one or more render targetsmay be transmitted to a display device in order to display the renderedimage.

By way of example, a set of render targets for the screen space 400could include eight render targets. The first render target couldinclude four fields representing color, including red, green, and bluecomponent colors, and transparency information associated with acorresponding fragment. The second render target could include twofields representing depth and stencil information associated with thecorresponding fragment. The third render target could include threefields representing surface normal vector information, including anx-axis normal vector, a y-axis normal vector, and a z-axis normalvector, associated with the corresponding fragment. The remaining fiverender targets could be configured to store additional informationassociated with the corresponding fragment. Such configurations couldinclude storage for various information, including, without limitation,3D positional data, diffuse lighting information, and specular lightinginformation.

Each cache tile 410 represents a portion of the screen space 400. Forclarity, only five cache tiles 410(0)-410(4) are shown in FIG. 4. Insome embodiments, cache tiles may have an arbitrary size in X and Yscreen space. For example, if a cache tile were to reside in a cachememory that also is used to store other data, then the cache tile couldbe sized to consume only a specific portion of the cache memory. Thesize of a cache tile may be based on a number of factors, including, thequantity and configuration of the render targets associated with thescreen space 400, the quantity of samples per pixel, and whether thedata stored in the cache tile is compressed. As a general matter, acache tile is sized to increase the likelihood that the cache tile dataremains resident in the cache memory until all graphics primitivesassociated with the cache tile are fully processed.

The raster tiles 420 represent a portion of the cache tile 410(0). Asshown, the cache tile 410(0) includes sixteen raster tiles420(0)-420(15) arranged in an array that is four raster tiles 420 wideand four raster tiles 420 high. In systems that include multiple GPCs208, processing associated with a given cache tile 410(0) may be dividedamong the available GPCs 208. In the example shown, if the sixteenraster tiles of cache tile 410(0) were processed by four different GPCs208, then each GPC 208 could be assigned to process four of the sixteenraster tiles 420 in the cache tile 410(0). Specifically, the first GPC208 could be assigned to process raster tiles 420(0), 420(7), 420(10),and 420(13). The second GPC 208 could be assigned to process rastertiles 420(1), 420(4), 420(11), and 420(14). The third GPC 208 could beassigned to process raster tiles 420(2), 420(5), 420(8), and 420(15).The fourth GPC 208 would then be assigned to process raster tiles420(3), 420(6), 420(9), and 420(12). In other embodiments, theprocessing of the different raster tiles within a given cache tile maybe distributed among GPCs 208 or any other processing entities includedwithin computer system 100 in any technically feasible manner.

Efficient Cache Management in a Tiled Architecture

FIG. 5 is a block diagram of a subsystem 500 configured to process pixeldata to generate pixels for display, according to one embodiment of thepresent invention. Subsystem 500 may be included within GPC 208 of FIG.2. In one embodiment, subsystem 500 is configured to implement certainfunctionality associated with graphics processing pipeline 350 discussedabove in conjunction with FIG. 3B, including that of tiling unit 375, aswell as other related functionality, as described in greater detailherein.

As shown, subsystem 500 includes a memory interface 510, a subsystemcontroller 520, a host interface 530, a fetch unit 540, and a pixelprocessing pipeline 550. Fetch unit 540 includes a surface cache 541, afetch controller 542, and a surface list 543. Pixel processing pipeline550 includes filtering units 560, scaling units 570, and blending units580. Memory interface 510, subsystem controller 520, fetch unit 540, andpixel processing pipeline 550 are coupled together via data connections580. Subsystem controller 520 and host interface 530 are also coupled toone another.

Memory interface 510 provides access to an external memory (not shownhere). The external memory could be a globally accessible memory, amemory surface included in a unified memory architecture, an L2 cache,or another type of memory unit. Subsystem controller 520 is amicrocontroller configured to manage the general functionality ofsubsystem 500 as a whole. Host interface 530 is configured to receiveinstructions from an external host (not shown) and to forward theinstructions to subsystem controller 520 for execution. Thoseinstructions may indicate different pixel-related processing operationsto perform with specific portions of pixel data.

Fetch unit 540 is configured to acquire pixel data for processing bypixel processing pipeline 550. Surface cache 541 within fetch unit 540is configured to cache pixel data on behalf of pixel processing pipeline550. Fetch controller 542 is configured to issue fetch requests tosurface cache 541 to acquire that pixel data. Surface list 543identifies the pixel data to be fetched and cached. Surface list 543includes a list of “surfaces” to be processed, where a surface generallyrefers to a cache tile and associated graphics data that may beprocessed to generate a screen tile of pixels.

In operation, surface list 543 sequentially steps through cache tilesassociated with a given world space frame and then indicates to fetchcontroller 542 which cache tiles should be acquired to generate acorresponding screen space frame. Fetch controller 542 then issues fetchrequests to surface cache 541 for graphics data associated with thosecache tiles, and surface cache 541 retrieves the graphics data fromexternal memory, as needed. The graphics data may include portions ofgeometry and/or graphics primitives, rasterized pixels in need offurther processing, and other types of graphics data. Pixel processingpipeline 550 may then access cached pixel data associated with thatgraphics data for processing.

Pixel processing pipeline 550 includes various processing units that maybe configured to process pixel data read from surface cache 541.Filtering units 560 may be configured to perform various filteringoperations, including, e.g., temporal noise filtering and cadencedetection, among others. Scaling units 570 may be configured to performvarious scaling operations, including, e.g., deinterlacing, Y-scaling,and X-scaling, among others. Blending units 580 may be configured toperform various types of blending operations, including colorconversions, among others. As a general matter, any of theaforementioned units of pixel processing pipeline 550 may be optionallyconfigured to perform different processing operations depending on theinstructions executed by subsystem controller 520.

As mentioned above, fetch unit 540 is configured to acquire graphicsdata, including pixel data, for processing by pixel processing pipeline550. In doing so, fetch unit 540 implements various techniques forstepping through cache tiles associated with a world space frame andcaching pixel data associated with each such cache tile, as described ingreater detail below in conjunction with FIGS. 6A-6B.

FIG. 6A is a conceptual diagram illustrating a pattern for accessingpixel data associated with a cache tile, according to one embodiment ofthe present invention. As shown, a world space frame 600 includes cachetiles 610(0) through 610(11), and a screen space frame 620 includesscreen tiles 630(0) through 630(11).

Each cache tile 610 of screen space frame 600 is associated with adifferent portion of graphics data, including pixel data in need offurther processing. Fetch unit 540 is configured to backproject worldspace frame 600 into screen space frame 620, meaning that fetch unit 540maps each cache tile 610 of world space frame 600 to a correspondingscreen tile 630 of screen space frame 620. In operation, surface list543 indicates particular cache tiles 610 to acquire for generatingcorresponding screen tiles 630. Then, fetch controller 542 determinesparticular cache lines within surface cache 541 where pixel dataassociated with screen tiles 630 may be stored for later access. Pixelprocessing pipeline 550 is configured to process each screen tile 630(0)through 630(11) to generate an image for display.

During the backprojection process mentioned above, surface list 543causes fetch controller 542 to acquire pixel data associated with eachcache tile 610 according to a particular sequence that is illustratedwithin world space frame 600 in FIG. 6A. In that sequence, cache tiles610 are accessed vertically from top to bottom along each column ofcache tiles 610. Those columns are then traversed horizontally from leftto right. This sequence is referred to herein as “vertical first.”Surface list 543 is configured to step through each cache tile 610 ofworld space frame 600 according to the vertical first sequence and toindicate to fetch controller 542 specific locations where pixel dataassociated with each cache tile 610 may be accessed. For each such cachetile 610, fetch controller 542 issues fetch requests to surface cache541 to acquire portions of pixel data needed to generate thecorresponding screen space tile 630. A given portion of the pixel datamay already reside within a given cache line of surface cache 541 (acache hit occurs), or surface cache 541 may need to access the portionof pixel data from external memory (a cache miss occurs). In eithercase, surface cache 541 eventually stores the needed pixel data.

A given portion of that pixel data may reside within the screen tile 630itself, and/or within a strip of pixel data associated with aneighboring screen tile 630. For example, in screen space frame 620,region 640(5) includes all portions of pixel data needed to processscreen tile 630(5), including strips of pixel data included withinneighboring screen tiles 630(0), 630(1), 630(2), 630(4), 630(6), 630(8),630(9), and 630(10). Similarly, region 640(6) includes all portions ofpixel data needed to process screen tile 630(6), including strips ofpixel data included within neighboring screen tiles 630(1), 630(2)630(3), 630(5), 630(7), 630(9), 630(10), and 630(11). A given strip mayinclude any number of columns or rows of pixels and associated data,depending on the particular pixel processing operations for which pixelprocessing pipeline 550 is configured to perform. For example, if pixelprocessing pipeline 550 is configured to implement a 5-tap filter, thena given strip would include two rows or columns of pixels associatedwith a neighboring screen tile 630.

Persons skilled in the art will notice that regions 640(5) and 640(6)overlap one another, indicating that some of the pixel data used forprocessing screen space tile 630(5) would also be needed for processingscreen space tile 630(6). Surface cache 541 is configured to storeportions of pixel data residing in such overlapping regions, allowingthat pixel data to be accessed multiple times when different screentiles are processed.

When surface cache 541 caches portions of pixel data associated with agiven screen tile 630, including portions of pixel data associated withneighboring tiles 630, fetch controller 542 assigns each differentportion to a particular cache line within surface cache 541. For eachsuch cache line, surface cache 541 assigns a “hint” value that indicatesthe likelihood that the pixel data stored at a given cache line will beneeded when a subsequent screen tile 630 is processed. This cachingstrategy is described in greater detail below in conjunction with FIG.6B.

Hint-Based Cache Policy to Reduce Thrashing of Surface Cache

FIG. 6B is a conceptual diagram illustrating a technique for cachingpixel data associated with a screen tile, according to one embodiment ofthe present invention. As shown, screen space frame 620 includes screenspace tiles 630(0) through 630(11), similar to FIG. 6A. Beneath screenspace frame 620, screen space tile 630(6) is shown, along with region640(6). Again, region 640(6) represents a region of pixel data that isneeded to process screen space tile 630(6), including pixel dataassociated with screen space tiles 630 that neighbor screen space tile630(6).

Fetch controller 542 is configured to assign different portions ofregion 640(6) to different cache lines 650. Accordingly, each cache line650 may store a different portion of pixel data used for processingscreen tile 630(6). When surface cache 541 stores those cache lines,surface cache 541 assigns a hint value to each such cache line based onthe position of that cache line 650 within region 640(6). As mentionedabove, the hint value for a given cache line 650 indicates thelikelihood that pixel data stored at that cache line will be needed whenprocessing a subsequent screen tile 630.

Due to the vertical first sequence with which fetch controller 542generates screen tiles 630, particular cache lines 650 are more likelyto be needed again than other cache lines. Specifically, cache lines 650associated with the bottom edge of a region 640 corresponding to a givenscreen tile 630 are likely to be needed for processing the top edge ofanother region 640 associated with the subsequent screen tile 630. Forexample, since cache lines 650 included in a horizontal strip 652 ofregion 640(6) overlap subsequent screen tile 630(7), those cache lines650 would be needed again for processing screen tile 630(7) afterprocessing of screen tile 630(6) is complete.

Since fetch controller 542 also traverses columns from left to right,cache lines 650 associated with the right edge of a region 640corresponding to a given screen tile 630 are likely to be needed forprocessing the left edge of another region 640 associated with asubsequent screen tile 630. For example, since cache lines 650 includedin a vertical strip 651 of region 640(6) overlap subsequent screen tile630(10), those cache lines 650 would be needed for processing screentile 630(10) after processing of screen tile 630(6) is complete.

Surface cache 541 is configured to assign higher hint values to cachelines associated with the bottom edge of a region 640 because thosecache lines are likely to be needed within a short interval of time(i.e., during the very next screen tile). Surface cache 541 is alsoconfigured to assign moderate hint values to cache lines associated withthe right-hand edge of a region 640 because those cache lines are likelyto be needed within a longer interval of time (i.e., after severalintervening screen tiles have been processed). As shown in FIG. 6B,surface cache 541 assigns a hint value of 2 to cache lines 650 withinhorizontal strip 652 and assigns a hint value of 1 to cache lines 650within vertical strip 651.

In addition, surface cache 541 assigns a hint value of 0 to cache lines650 that reside within an interior subregion of region 640 and does notoverlap other adjacent regions 640, as is shown. A hint level of zeroindicates that the associated cache lines are not likely to be neededfor processing subsequent screen tiles 630. As a general matter, surfacecache 541 is configured to assign different hint values to differentcache lines that store pixel data associated with different subregionsof a region 640. Those subregions may include edges or interiorsubregions of region 640, among other possibilities. In any case, thehint value associated with a cache line falling within a given subregionindicates whether the pixel data stored by the cache line will be neededagain, and how soon that pixel data will be needed (if at all). Personsskilled in the art will recognize that the particular hint valuesdiscussed herein may be configurable based on the order with whichscreen tiles are traversed, among other things.

Surface cache 541 relies upon the hint values described above whenimplementing an eviction policy. Generally, surface cache 541 implementsa most recently used (MRU) policy that is modified to reflect that cachelines 650 with lower hint values should be evicted before cache lines650 with higher hint values. With this approach, surface cache 541 mayretain cache lines 650 that are likely to be needed again and evictcache lines 650 that may not be needed again.

At any given point in time, surface cache 541 may include cache lines650 having various different hint values. Those cache lines 650 may alsobe locked or unlocked. When a surface cache 541 receives a fetch requestfrom fetch controller 542 for pixel data that is resident within a givencache line 650 (a cache hit occurs), then surface cache 541 locks thatcache line 650 to prevent eviction, thereby allowing pixel processingpipeline 550 to safely access the corresponding pixel data.Alternatively, when surface cache 541 receives a fetch request fromfetch controller 542 for pixel data that is not resident within anycache line 650 (a cache miss occurs), then surface cache 541 fetches theneeded pixel data from external memory. Surface cache 541 caches thefetched pixel data, evicting an occupied cache line as needed. In doingso, surface cache 541 may evict any unlocked cache line according to themodified MRU policy mentioned above and then lock the evicted cache lineto store incoming, fetched pixel data. When the pixel data is received,surface cache 541 stores the data in the recently evicted and lockedcache line. A given cache line 650 remains locked until pixel processingpipeline 550 has read out all pixel data from the cache line that isneeded for processing the current screen tile 630. The caching policyimplemented by surface cache 541 is described in stepwise fashion belowin conjunction with FIG. 7.

FIG. 7 is a flow diagram is method steps for managing pixel data in asurface cache during processing of a tile, according to one embodimentof the present invention. Although the method steps are described inconjunction with the systems of FIGS. 1-5, persons skilled in the artwill understand that any system configured to perform the method steps,in any order, is within the scope of the present invention.

As shown, a method 700 begins at step 701, where surface cache 541receives a fetch request from fetch controller 542 for a portion ofpixel data. The portion of pixel data is associated with a screen tile630 to be processed by pixel processing pipeline 550. At step 702,surface cache 541 determines whether a cache line of surface cache 541includes the request pixel data. If the requested portion of pixel datais resident, then a cache hit occurs, and surface cache 541 proceeds tostep 707, as described in greater detail below.

Otherwise, if surface cache 541 determines at step 702 that therequested portion of pixel data is not resident, then surface cache 541proceeds to step 703. At step 703, surface cache 541 fetches therequested portion of pixel data from external memory. At step 704,surface cache 542 evicts data from a cache line, according to themodified MRU policy described above, to create space for the fetcheddata, as needed. In doing so, surface cache 542 locks the recentlyevicted cache line to reserve that cache line for pixel data to befetched and sets a valid bit to false. At step 705, surface cache 542stores the fetched portion of pixel data in the available, locked cacheline and sets the valid bit to true, indicating that the cache line nowstores valid data. At step 706, surface cache 542 assigns a hint levelto the cache line based on the position of the portion of pixel datawithin the current screen tile. One technique for assigning the hintvalue is described in detail above in conjunction with FIG. 6B. However,persons skilled in the art will understand that any technique forassigning a hint value to a cache line based on the position of thecache line within a screen tile falls within the scope of the invention.

At step 707, surface cache 542 locks the cache line associated with theportion of pixel data, as needed. Surface cache 542 performs step 707unless the cache line has already been locked in performing step 704.Locking the cache line in this fashion prevents the cache line frombeing evicted before the pixel data stored therein can be read out topixel processing pipeline 550. At step 708, surface cache 541 determinesthat pixel processing pipeline 550 is ready to process the portion ofpixel data. At step 709, surface cache 541 reads out the portion ofpixel data to pixel processing pipeline 550. At step 710, surface cache541 unlocks the cache line, thereby enabling that cache line to beevicted at a later time. The method 700 then ends.

By implementing the hint-based cache policy described herein, surfacecache 541 may limit the eviction of cache lines that will be neededagain. In particular, cache lines associated with pixel data needed forprocessing subsequent screen tiles are assigned higher hint values thancache lines associated with portions of pixel values that may not beneeded for processing subsequent screen tiles. As such, surface cache541 may operate with greater efficiency compared to prior artapproaches.

Fetch controller 542 may improve the efficiency of surface cache 542 bythrottling the issuance of fetch requests under specific circumstances,thereby preventing premature eviction of cache lines having higher hintvalues, as described in greater detail below.

Throttling Fetch Requests to Prevent Premature Eviction

When acquiring pixel data for a current tile, fetch controller 542 iscapable of issuing fetch requests to surface cache 541 to acquire pixeldata at a much higher rate than pixel processing pipeline 550 is capableof reading out cached pixel data for processing. In particular, fetchcontroller 542 is capable of issuing a fetch request for 256 bytes ofpixel data every 4 clock cycles, while pixel processing pipeline 550 isonly capable of processing several bytes of pixel data per clock cycle.A consequence of this bandwidth mismatch is that surface cache 542 maybe forced to evict unlocked cache lines with higher hint values in orderto create space for incoming pixel data associated with a fetch request.As mentioned above, cache lines with higher hint values are likely to beneeded when subsequent screen tiles are processed. However, if thosecache lines are evicted, the pixel data associated with those cachelines would need to be re-fetched, an occurrence commonly known a cachetrashing.

To prevent such thrashing, fetch controller 542 is configured to stopissuing fetch requests when surface cache 541 stores sufficient pixeldata to supply pixel processing pipeline 550 and does not include anyunlocked cache lines with low hint values. In particular, when surfacecache 541 includes a threshold number of locked cache lines and zerounlocked cache lines with a hint value of 0, then fetch controller 542stops issuing fetch requests. With this approach, surface cache 541 isprevented from evicting cache lines with higher hint values when nounlocked cache lines with a hint value of 0 are available for eviction.

Further, when cache 541 includes the threshold number of locked cachelines, pixel processing pipeline 550 still has access to sufficientpixel data for processing. As pixel processing pipeline 550 drainssurface cache 541, additional cache lines 650 will become unlocked, andsome of those cache lines will have a hint level of zero. Those cachelines may be safely evicted to create space for additional pixel data tobe fetched. When a cache line 650 with a hint value of 0 is unlocked,then fetch controller 542 may continue issuing fetch requests. Theunlocked hint level 0 cache line may be safely evicted, and pixel dataassociated with a new fetch request may be stored in the recentlyevicted cache line. When the number of locked cache lines falls beneaththe threshold value, fetch controller 542 may likewise continue issuingfetch requests in order to supply pixel processing pipeline 550 withsufficient pixel data to continue processing the current screen tile.

The threshold number of locked cache lines described above is aconfigurable value that may be derived from the size and dimension ofscreen tiles 630, among other factors. In practice, a microcontrollersets the threshold based on various operational parameters, including ascaling ratio, memory latency values, and other parameters. Themicrocontroller may also adjust the threshold on a per-frame basisdepending on the specific tiling configuration associated with eachdifferent frame.

By throttling fetch requests in the fashion described herein, fetchcontroller 542 is capable of reducing thrashing of surface cache 541.Accordingly, surface cache 541 may efficiently implement the modifiedMRU policy described above in conjunction with FIGS. 6B-7 without beingforced to evict cache lines with higher hint values. However, thetechnique described herein may also be implemented independently of themodified MRU policy. In particular, fetch controller 542 may throttlefetch requests specifically based on the screen position associated witheach stored cache line, and may not rely on the hint mechanism describedthus far. One technique for throttling fetch requests that may beimplemented by fetch controller 542 is described in stepwise fashionbelow in conjunction with FIG. 8.

FIG. 8 is a flow diagram of method steps for throttling fetch requestsissued to a surface cache, according to one embodiment of the presentinvention. Although the method steps are described in conjunction withthe systems of FIGS. 1-5, persons skilled in the art will understandthat any system configured to perform the method steps, in any order, iswithin the scope of the present invention.

As shown, a method 800 begins at step 801, where fetch controller 542identifies pixel data that is needed to process the current tile. Fetchcontroller 542 may interact with surface list 543 to identify allportions of pixel data needed to generate a screen tile from the currentcache tile. At step 802, fetch controller 542 communicates with surfacecache 541 to determine whether all needed pixel data is currentlyresident in that cache. In particular, fetch controller 542 determines,at step 802, whether pixel processing pipeline 550 is capable completingthe processing of the current screen tile, in its entirety, with thepixel data that is currently resident within surface cache 541, i.e.,without acquiring additional pixel data. If surface cache 541 stores allsuch needed data, then the method 800 ends.

However, if fetch controller 542 determines at step 802 that additionalpixel data is needed, then at step 803, fetch controller 542 issues afetch request to surface cache 541. Surface cache 541 may respond byimplementing the method 700 described above in conjunction with FIG. 7.In doing so, surface cache 541 may process a cache hit or a cache missand then store the requested pixel data. As also discussed above,surface cache 541 may store pixel data in different cache lines withspecific hint values that indicate the likelihood that the stored pixeldata will be needed when processing subsequent cache tiles to generatecorresponding screen tiles. However, as also noted, surface cache 541could potentially be forced to evict unlocked cache lines with higherhint values when receiving fetch requests from fetch controller 542 fora large amount of pixel data. Again, this situation may arise becausefetch controller 542 is capable of issuing fetch requests for additionalpixel data much faster than pixel processing pipeline 550 can drainpixel data from surface cache 541.

To mitigate this issue, at step 804, fetch controller 542 determineswhether surface cache 541 includes a threshold number of locked cachelines and zero unlocked cache lines with a hint value of zero. Ifneither of these conditions are true, then fetch controller 542 returnsto step 802 and proceeds as described above. Otherwise, fetch controller542 proceeds to step 805 and stops issuing fetch requests to surfacecache 541, thereby avoiding the eviction of cache lines with higher hintvalues.

At step 806, fetch controller 542 allows pixel processing pipeline 550to drain surface cache 541. Since surface cache 541 includes thethreshold number of locked cache lines 650, pixel processing pipeline550 still has access to sufficient pixel data for processing. Whendraining surface cache 541 at step 806, pixel processing pipeline 550may read out all pixel data associated with a given cache line having ahint level of zero, and that cache line may then become unlocked andeligible for eviction. Fetch controller returns 542 returns to step 804after draining pixel data from surface cache 541 and proceeds asdescribed above. Steps 804 through 806 may be repeated until surfacecache 541 does not include the threshold number of locked cache lines,indicating that pixel processing pipeline 550 needs additional pixeldata that is not resident in surface cache 541. Steps 804 through 806may also be repeated until certain cache lines with a hint level of zerohave become unlocked and therefore eligible for safe eviction.

By implementing the method 800, fetch controller 542 may avoidsituations where the bandwidth mismatch between pixel data fetch rateand pixel data drain rate results in premature evictions from surfacecache 541, thereby enhancing the efficiency of surface cache 541.

Referring generally to FIGS. 5-8, persons skilled in the art willrecognize that the hint-based cache policy described in conjunction withFIGS. 6A-7 and the fetch request throttling technique described inconjunction with FIG. 8 may be implemented by subsystem 500 of FIG. 5 inconjunction with one another. However, those skilled in the art willalso understand that either approach may be implemented separately andindependently of one another.

For example, surface cache 541 could implement the hint-based cachingpolicy without the benefit of fetch controller 542 throttling fetchrequests. Likewise, fetch controller 542 could throttle fetch requestsbased on other criteria aside from that discussed in conjunction withstep 804. Generally, fetch controller 542 need only determine thatsurface cache 541 stores sufficient data for pixel processing pipeline550 and that issuing additional fetch requests would cause prematureeviction of pixel data needed for later processing. Fetch controller 541could, in practice, make such a determination regardless of whethersurface cache 541 implements the hint-based policy described herein.

In sum, a surface cache stores pixel data on behalf of a pixelprocessing pipeline configured to process the pixel data to generatescreen tiles. The surface cache assigns hint levels to cache linesstoring portions of pixel data according to whether those portions ofpixel data are likely to be needed again. When a portion of pixel datais associated with certain edges of a current screen tile, that portionmay be needed to process a subsequent tile and is therefore assigned ahigher hint value. When the portion of pixel data is associated with acentral area of the current tile, that portion may not be needed againand is assigned a lower hint value. The surface cache is configured topreferentially evict cache lines having a lower hint value, therebypreserving cache lines that may be needed again and improving cacheefficiency. In addition, a fetch controller is configured to throttlethe rate at which fetch requests are issued to the surface cache inorder to prevent situations where pixel data that is likely to be neededagain becomes prematurely evicted, further improving cache efficiency.

One advantage of the techniques disclosed herein is that surface cacheefficiency is improved because pixel data that will be needed again isretained within the cache. Another advantage is that additional pixeldata are not fetched until cache lines that are no longer necessary areunlocked, thereby avoiding situations where pixel data needed for futureoperations is prematurely evicted.

One embodiment of the invention may be implemented as a program productfor use with a computer system. The program(s) of the program productdefine functions of the embodiments (including the methods describedherein) and can be contained on a variety of computer-readable storagemedia. Illustrative computer-readable storage media include, but are notlimited to: (i) non-writable storage media (e.g., read-only memorydevices within a computer such as compact disc read only memory (CD-ROM)disks readable by a CD-ROM drive, flash memory, read only memory (ROM)chips or any type of solid-state non-volatile semiconductor memory) onwhich information is permanently stored; and (ii) writable storage media(e.g., floppy disks within a diskette drive or hard-disk drive or anytype of solid-state random-access semiconductor memory) on whichalterable information is stored.

The invention has been described above with reference to specificembodiments. Persons of ordinary skill in the art, however, willunderstand that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The foregoing description and drawingsare, accordingly, to be regarded in an illustrative rather than arestrictive sense.

Therefore, the scope of embodiments of the present invention is setforth in the claims that follow.

The invention claimed is:
 1. A subsystem configured to cache data in atile-based architecture, comprising: a fetch controller configured toissue requests for portions of data; and a cache unit coupled to thefetch controller and configured to: receive a request for a firstportion of data corresponding to a first subregion associated with ascreen tile from the fetch controller, determine that the first portionof data is not resident in the cache unit, acquire the first portion ofdata from a memory unit, identify, based on a cache policy, a firstcache line in the cache unit that stores a second portion of datacorresponding to a second subregion associated with the screen tile,wherein the cache policy indicates that one or more portions of datacorresponding to the second subregion should be evicted before one ormore portions of data corresponding to other subregions associated withthe screen tile are evicted, evict the second portion of data from thefirst cache line, and after evicting, store the first portion of data inthe first cache line.
 2. The subsystem of claim 1, wherein the cacheunit is configured to store the first portion of data in the first cacheline by assigning a first value to the first cache line that exceeds asecond value previously assigned to the first cache line when the firstcache line stored the second portion of data.
 3. The subsystem of claim2, wherein the first value is associated with the first subregion, andthe second value is associated with the second subregion, and the cacheunit is further configured to: assign the first value to one or morecache lines in the cache unit that store portions of data correspondingto the first subregion; and assign the second value to one or more cachelines in the cache unit that store portions of data corresponding to thesecond subregion.
 4. The subsystem of claim 1, wherein the firstsubregion comprises an edge subregion associated with the screen tile,and the second subregion comprises an interior subregion associated withthe screen tile.
 5. The subsystem of claim 1, wherein the firstsubregion comprises a vertical edge subregion associated with the screentile, and the second subregion comprises a horizontal edge subregionassociated with the screen tile.
 6. The subsystem of claim 1, whereinthe cache unit is further configured to: lock the first cache line toprevent data stored in the first cache line from being evicted; read thefirst portion of data from the first cache line to transmit the firstportion of data to a processing pipeline; and unlock the first cacheline for the data stored in the first cache line to be evicted.
 7. Thesubsystem of claim 6, wherein the cache unit is further configured to:receive another request to access the first portion of data from thefirst cache line; and lock the first cache line to prevent the datastored in the first cache line from being evicted.
 8. The subsystem ofclaim 1, wherein the first subregion and the second subregion residewithin a region of a frame that includes pixel data associated with thescreen tile and pixel data associated with other screen tiles residingadjacent to the screen tile in the frame.
 9. A computer-implementedmethod for caching data in a tile-based architecture, the methodcomprising: receiving a request for a first portion of datacorresponding to a first subregion associated with a screen tile;determining that the first portion of data is not resident in a cacheunit; acquiring the first portion of data from a memory unit;identifying, based on a cache policy, a first cache line in a cache unitthat stores a second portion of data corresponding to a second subregionassociated with the screen tile, wherein the cache policy indicates thatone or more portions of data corresponding to the second subregionshould be evicted before one or more portions of data corresponding toother subregions associated with the screen tile are evicted; evictingthe second portion of data from the first cache line; and afterevicting, storing the first portion of data in the first cache line. 10.The computer-implemented method of claim 9, wherein storing the firstportion of data in the first cache line further comprises assigning afirst value to the first cache line that exceeds a second valuepreviously assigned to the first cache line when the first cache linestored the second portion of data.
 11. The computer-implemented methodof claim 10, wherein the first value is associated with the firstsubregion, and the second value is associated with the second subregion,and further comprising: assigning the first value to one or more cachelines in the cache unit that store portions of data corresponding to thefirst subregion; and assigning the second value to one or more cachelines in the cache unit that store portions of data corresponding to thesecond subregion.
 12. The computer-implemented method of claim 9,wherein the first subregion comprises an edge subregion associated withthe screen tile, and the second subregion comprises an interiorsubregion associated with the screen tile.
 13. The computer-implementedmethod of claim 9, wherein the first subregion comprises a vertical edgesubregion associated with the screen tile, and the second subregioncomprises a horizontal edge subregion associated with the screen tile.14. The computer-implemented method of claim 9, further comprising:locking the first cache line to prevent data stored in the first cacheline from being evicted; reading the first portion of data from thefirst cache line to transmit the first portion of data to a processingpipeline; and unlocking the first cache line the data stored in thefirst cache line to be evicted.
 15. The computer-implemented method ofclaim 14, further comprising: receiving another request to access thefirst portion of data from the first cache line; and locking the firstcache line to prevent the data stored in the first cache line from beingevicted.
 16. The computer-implemented method of claim 9, wherein thefirst subregion and the second subregion reside within a region of aframe that includes pixel data associated with the screen tile and pixeldata associated with other screen tiles residing adjacent to the screentile in the frame.
 17. A computing device configured to cache data in atile-based architecture, comprising: a memory unit; a fetch controllerconfigured to issue requests for portions of data; and a cache unitcoupled to the memory unit and to the fetch controller and configuredto: receive a request for a first portion of data corresponding to afirst subregion associated with a screen tile from the fetch controller,determine that the first portion of data is not resident in the cacheunit, acquire the first portion of data from the memory unit, identify,based on a cache policy, a first cache line in the cache unit thatstores a second portion of data corresponding to a second subregionassociated with the screen tile, wherein the cache policy indicates thatone or more portions of data corresponding to the second subregionshould be evicted before one or more portions of data corresponding toother subregions associated with the screen tile are evicted, evict thesecond portion of data from the first cache line, and after evicting,store the first portion of data in the first cache line.
 18. Thecomputing device of claim 17, wherein the cache unit is configured tostore the first portion of data in the first cache line by assigning afirst value to the first cache line that exceeds a second valuepreviously assigned to the first cache line when the first cache linestored the second portion of data.
 19. The computing device of claim 18,wherein the first value is associated with the first subregion, and thesecond value is associated with the second subregion, and the cache unitis further configured to: assign the first value to one or more cachelines in the cache unit that store portions of data corresponding to thefirst subregion; and assign the second value to one or more cache linesin the cache unit that store portions of data corresponding to thesecond subregion.
 20. The computing device of claim 17, wherein thefirst subregion comprises an edge subregion associated with the screentile, and the second subregion comprises an interior subregionassociated with the screen tile.